50??!! holy crap, do you have water left in there? or just fish? lol jk Anyway, when you say Goldfish
, do you mean those feeders you see in petco that have like 200 fish in a 50 gallon tank? If so, i would just give a lot of them away to hobbyists that have predatory fish. However, if you have the more common bulbous Goldfish
, theya re poop machiens and the numbers must be reduced by store returns. I am agaisnt euthanizing fish, as thats very... wrong, they all deserve to live IMO. Even if you got the msot powerful canister, with 50 Goldfish
, you would still be cleaning it once every 2 days. Also the stress of overpopulation will certainly kill off a huge number of them, so it will be better to mvoe them now isntead of letting them all suffer. Moreover, this kind of overpopulation may spark diseases that could just decimate your tank.
If they are fancy Goldfish
and there is really no way to reduce the numbers, then a new tank is another potential solution except it super expensive to do this... since you would need a tank the size of... a pond. Or if you must keep every fish and really MUST just get a filter then ur out of luck. I dont think anything short of a pond filter could handle that many fancy Goldfish
Ultimatly your questions still need to be answered.
the 405 would create too powerful of a current in 40 gallons and your fish will be blown around. There areoutput diverters you can get as well as these tube things that will create multiple outputs so less pressure in one spot.
You do nt ahve the option of using the 305 since you have too many fish, actually even the 405 is not adaquate but if you must pick between these 2 the 405 is better and the extra filtration is justified becuase it puts the water through an extra set of baskets with media and will improve your biological filtration by a lot.
Yes the 405 is a way beter choice for your situation. However again, the "best" choice will be to reduce your population.